from what i have heard as of late it goes on the weight of the car. if the car is under 1100kg, multiply the weight by 2.5 and thats your engine capacity. if it weighs over 1100 you multiply the weight by 4. for example my rt118 corona wagon weighs in at 1260kg curb weight, which means i can bolt anything up to approximately 5.0 litre. and from memory if you want to turbocharge or supercharge, you have to multiply the engine capacity of the motor you want to put in by 1.8....methinks?